Borrelia burgdorferi is the etiologic agent of Lyme disease, a tick-borne spirochetosis. In the wild, the spirochete cycles between the tick vector and deer mice or other mammals. Humans come in contact with the spirochete when they are bitten by an infected tick. When left untreated, the disease may become chronic, culminating in debilitating arthritis, subacute encephalopathy, and neurologic disease. During the development of clinical presentations, vigorous cellular and humoral immune responses ensue which, for unclear reasons, are unable to eradicate the invading spirochetes. The ability of B. burgdorferi to cause persistent infection has prompted efforts to define the mechanisms underlying the bacterium?s remarkable immuno- evasivness. In addition, identification of the mechanisms that allow the spirochete to cycle between the widely different host environments, such as the tick and the mammal, would be important with regards to understanding the biology of this enigmatic pathogen. Given the premise that gene redundancy in pathogenic organisms may be driven by powerful selective pressures, perhaps related to the need for B. burgdorferi to exist in diverse host environments, or to avoid a prolonged immune response, I have focused my research efforts on a family of lipoproteins in this spirochete. I have discovered a large family of potential surface-exposed lipoproteins whose genes reside on many (<7) different, yet highly similar co-migrating 30 kb plasmids in B. burgdorferi strain 297. I am now in the process of cloning, sequencing and characterizing these proteins from strain B31. In strain B31 the circular plasmid profile has been identified for several passage variants. Having these passage variants, which contain different numbers and types of circular plasmids, allows us the opportunity to study the role of expression of the genes encoding these proteins under different plasmid population backgrounds while under different environmental influences. In addition, efforts to determine the function of these lipoproteins are also underway.